206 research outputs found
Magnetic Fluctuations, Precursor Phenomena and Phase Transition in MnSi under Magnetic Field
The reference chiral helimagnet MnSi is the first system where skyrmion
lattice correlations have been reported. At zero magnetic field the transition
at to the helimagnetic state is of first order. Above , in a region
dominated by precursor phenomena, neutron scattering shows the build up of
strong chiral fluctuating correlations over the surface of a sphere with radius
, where is the pitch of the helix. It has been suggested that
these fluctuating correlations drive the helical transition to first order
following a scenario proposed by Brazovskii for liquid crystals. We present a
comprehensive neutron scattering study under magnetic fields, which provides
evidence that this is not the case. The sharp first order transition persists
for magnetic fields up to 0.4 T whereas the fluctuating correlations weaken and
start to concentrate along the field direction already above 0.2 T. Our results
thus disconnect the first order nature of the transition from the precursor
fluctuating correlations. They also show no indication for a tricritical point,
where the first order transition crosses over to second order with increasing
magnetic field. In this light, the nature of the first order helical transition
and the precursor phenomena above , both of general relevance to chiral
magnetism, remain an open question
Real-space observation of quasicrystalline Sn monolayer formed on the fivefold surface of icosahedral AlCuFe quasicrystal
We investigate a thin Sn film grown at elevated temperatures on the fivefold surface of an icosahedral AlCuFe quasicrystal by scanning tunneling microscopy (STM). At about one monolayer coverage, the deposited Sn is found to form a smooth film of height consistent with one-half of the lattice constant of the bulk Sn. Analysis based on the Fourier transform and autocorrelation function derived from high-resolution STM images reveals that Sn grows pseudomorphically and hence exhibits a quasicrystalline structure
Growth of Bi thin films on quasicrystal surfaces
We present a comprehensive study of Bi thin-film growth on quasicrystal surfaces. The substrates used for the growth are the fivefold surface of icosahedral (i)-Al-Cu-Fe and i-Al-Pd-Mn and the tenfold surface of decagonal (d)-Al-Ni-Co quasicrystals. The growth is investigated at 300 and 525 K substrate temperatures and at different coverage (θ) ranging from submonolayer to ten monolayers. The film is characterized by scanning tunneling microscopy, reflection high-energy electron diffraction, and x-ray photoelectron spectroscopy. At 300 K, the deposited Bi yields a quasicrystalline film for θ≤1. For 1\u3cθ\u3c5, it forms nanocrystallites with (100) surface orientation. The islands have magic heights, which correspond to the stacking of four atomic layers (predominantly). The selection of magic heights is interpreted in terms of quantum size effects arising from the electron confinement within the film thickness. The islands establish rotational epitaxial relationship with the substrate. For higher coverage, the film grows with monatomic height, not with magic heights, and reflects the symmetry of the bulk Bi. When deposition is performed at 525 K, terrace diffusion is more effective, resulting in the aggregation of Bi adatoms developing into a smooth monolayer with quasiperiodic order. At this temperature, multilayers do not adsorb
Quasicrystalline nature of quasicrystal surfaces: A photoemission study
Differently prepared surfaces of quasicrystalline i-Al-Pd-Mn are analyzed using angle-resolved photoemission in the x-ray andultraviolet range of photon energies. Depending on the preparation, we find both surfaces with crystalline structure and metallic character, and surfaces with quasicrystalline structural fingerprints and a suppressed density of states at the Fermi level, compatible with a pseudogap
Morphology of Sn Films Grown on the Fivefold Surface of Icosahedral Al63Cu24Fe13 Investigated by Scanning Tunneling Microscopy
Sn film growth on the fivefold surface of icosahedral Al63Cu24Fe13 has been investigated by employing scanning tunneling microscopy. For about monolayer coverage, the deposited Sn forms a layer of monoatomic height. A Fourier transform of the layer\u27s structure reveals quasicrystalline long range order. At higher coverage, flat-topped clusters of uniform heights are formed. The clusters preferentially grow at the step edges
Structure of the Fivefold Surface of the Icosahedral Al-Cu-Fe Quasicrystal: Experimental Evidence of Bulk Truncations at Larger Interlayer Spacings
Based on scanning tunneling microscopy of the fivefold surface of the icosahedral Al-Cu-Fe quasicrystal and the refined structure model of the isostructural i-Al-Pd-Mn, we present evidence that the surface corresponds to bulk truncations at the positions where blocks of atomic layers are separated by larger interlayer spacings (gaps). Both step-height distribution and high resolution scanning tunneling microscopy images on terraces reveal bulk truncations at larger gaps
Surface oxidation of a quasicrystalline Al–Cu–Fe alloy: No effect of surface orientation and grain boundaries on the final state
We have used x-ray photoelectron spectroscopy and Auger electron spectroscopy to examine the characteristics of oxides on two types of quasicrystalline Al–Cu–Fe samples. One type was formed by consolidation of powders, resulting in multiple grains with random surface orientations. The other was a single grain, oriented to expose a fivefold surface. Both were oxidized to saturation in a variety of environments at room temperature. We measured the elemental constituents that oxidized, the extent of oxygen-induced Al segregation, and the depth of the oxide. Under the conditions of our experiments, there was little, if any, significant difference between the two types of samples. Hence, surface orientation and bulk microstructure played little or no role on the final state of the oxide under these conditions
Magnetic and structural transitions in LaNaFeAs single crystals
LaNaFeAs single crystals have been grown out of an
NaAs flux in an alumina crucible and characterized by measuring magnetic
susceptibility, electrical resistivity, specific heat, as well as single
crystal x-ray and neutron diffraction. LaNaFeAs single
crystals show a structural phase transition from a high temperature tetragonal
phase to a low-temperature orthorhombic phase at T\,=\,125\,K. This
structural transition is accompanied by an anomaly in the temperature
dependence of electrical resistivity, anisotropic magnetic susceptibility, and
specific heat. Concomitant with the structural phase transition, the Fe moments
order along the \emph{a} direction with an ordered moment of
0.7(1)\, at \emph{T}\,=\,5 K. The low temperature stripe
antiferromagnetic structure is the same as that in other
\emph{A}FeAs (\emph{A}\,=\,Ca, Sr, Ba) compounds.
LaNaFeAs provides a new material platform for the
study of iron-based superconductors where the electron-hole asymmetry could be
studied by simply varying La/Na ratio.Comment: 9 pages, 7 figures, to appear in Physical Review
Lattice and spin dynamics in bcc Fe, 10 at. % Be
Body centered cubic Fe1−xBex alloys are known to display an enhanced tetragonal magnetostriction compared to Fe. In order to characterize the enhanced magnetoelasticity observed in this alloy system, we present detailed inelastic neutron scattering measurements of the phonon dispersion relations of a single crystal of bcc Fe, 10 at. %Be along the high symmetry directions [100], [110], and [111] at room temperature. We observe in particular that the frequency of transverse phonons propagating along the [110] direction with a [10] polarization at the zone boundary is reduced by 10% with respect to bcc Fe (the corresponding elastic shear constant c′ = ½(c11−c12) associated with this mode is approximately 70% that of pure Fe). The dispersion of spin waves has also been determined for energy transfers up to 40 meV and is found to follow the isotropic dispersion relation E(q) = Dq2, with D ∼ 200 meV/Å2 [for Fe, E(q) = Dq2, with D = 280 meV/Å2]
Interplay between Fe and Nd magnetism in NdFeAsO single crystals
The structural and magnetic phase transitions have been studied on NdFeAsO
single crystals by neutron and x-ray diffraction complemented by resistivity
and specific heat measurements. Two low-temperature phase transitions have been
observed in addition to the tetragonal-to-orthorhombic transition at T_S = 142
K and the onset of antiferromagnetic (AFM) Fe order below T_N = 137 K. The Fe
moments order AFM in the well-known stripe-like structure in the (ab) plane,
but change from AFM to ferromagnetic (FM) arrangement along the c direction
below T* = 15 K accompanied by the onset of Nd AFM order below T_Nd = 6 K with
this same AFM configuration. The iron magnetic order-order transition in
NdFeAsO accentuates the Nd-Fe interaction and the delicate balance of c-axis
exchange couplings that results in AFM in LaFeAsO and FM in CeFeAsO and
PrFeAsO.Comment: revised; 4 pages, 3 figures; accepted for publication in Phys. Rev.
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